Method and device for wavelet denoising

ABSTRACT

The present invention provides for a digital receiver arrangement and related method, in which a digital receiver arrangement comprises a tuner/demodulator circuit, an analogue-to-digital converting means, and further includes means for storing an impulse wavelet representation, means for determining if an interference impulse is present in a received signal, and means for introducing the stored representation of the impulse wavelet to a detected received impulse so as to counteract the effect thereof within the received signal.

The present invention relates to a digital receiver and related methodof receiving digital signals.

Digital broadcast systems such as the Coded Orthogonal FrequencyDivision Multiplexing (COFDM) have found particular use in Digital AudioBroadcasting (DAB) and Digital Terrestrial Television (DVB-T).

In such a broadcast system, the bits of each digital sample aredistributed over a number of carriers and at a number of different timeintervals. The carriers are each at a different evenly-spaced frequencywithin the transmission channel. The waveform in each time interval isnormally created by an Inverse Fast Fourier Transform (IFFT) of theamplitude and phase values required for the carriers during that timeinterval.

The output of each IFFT creates a so-called symbol, which has durationof a symbol period. A plurality of such symbols are combined so as toform a COFDM frame. The COFDM frames are then transmitted and thedistribution of bits of data across frequency and time provides for asystem which is advantageously error resistant and can also cope with ahigh degree of multipath distortion.

In the receiver's demodulator a Fast Fourier Transform (FFT) isperformed on the waveform in each symbol to determine the amplitude andphase of the carriers of which it is comprised. From these amplitude andphase values the data carried by the symbol is derived.

COFDM is a version of a communications system referred to as OrthogonalFrequency Division Multiplexing (OFDM). The features of which aredescribed in greater detail in European Standard EN 300 744, V1.1.2(1997-8), Digital Video Broadcasting (DVB); Framing Structure, channelcoding and modulation for digital terrestrial television, published byEuropean Telecommunications Standard Institute (ETSI) Valbonne, France,August 1997.

Disadvantages do however arise in that the sequentially transmittedsymbols can be severely affected by short term noise events that canserve to distort each individual symbol. For example, COFDM basedsystems such as those discussed above, can be disrupted by stronginterference from traffic-related switching events or high currentswitching events arising within domestic environments.

It would therefore be advantageous to try and limit the effect of noise,such as impulsive interference events, within digital signaltransmission systems.

EP-A-1043874 discloses a COFDM receiver arrangement that seeks to limitthe effect of impulsive interference events and in which the magnitudeof a received signal is compared to threshold levels in an attempt toidentify the presence of impulsive interference. If such threshold valueis exceeded, then that portion of the signal exceeding the threshold isreplaced by a zero value in an attempt to limit the effect of theinterference.

However, such a system is disadvantageously limited insofar as itsoperation depends upon a mere comparison of magnitude and thresholdvalue and so an inaccurate determination of the presence of impulsiveinterference events can prove common. Also, the mere replacement of alikely interference impulse merely by a zero value does not necessarilylead to a fully accurate recreation of the original signal absent to theinterference impulse.

The present invention therefore seeks to provide for a digital receiverarrangement, and related method, having advantages over known sucharrangements and methods.

According to a first aspect of the present invention, there is provideda digital receiver arrangement comprising a tuner/demodulator circuitand analogue-to-digital converting means, and further including meansfor storing an impulse wavelet representation, means for determining ifan interference impulse is present in a received signal, and means forintroducing the stored representation of the impulse wavelet to thedetected received impulse so as to counteract the effect thereof withinthe received signal.

The invention is advantageous in providing means for accurately seekingto remove an interference impulse from an incoming digital signal and ina manner having limited effect on the remainder of the signal.

The feature of Claim 2 represents a particularly accurate and efficientmeans for determining the arrival of an impulse interference eventwithin a received signal and the features of the Claims 3 and 4 compriseadvantageous embodiments of such means.

The feature of Claim 5 is advantageous in offering a particularlysimple, and effective means serving to remove the impulse interferenceevent from the received signal.

The feature of Claim 6 proves particularly advantageous in generating astored representation of an impulse infringement event that can offer ahigh degree of accuracy when compared with an incoming impulseinterference event in the received signal.

The features of Claim 10 and 11 prove particularly advantageous inenhancing the accuracy with which an impulse interference event can beeffectively removed from the received incoming digital signal.

According to another aspect of the present invention, there is provideda method of receiving a digital signal including the steps ofdemodulating the signal, and conducting an analogue-to-digitalconversion of the signal, and including the further steps of storing animpulse wavelet representation, determining if an impulse interferenceevent is found within an incoming signal, and introducing the saidstored wavelet representation to the received impulse interference eventso as to counteract the effect thereof.

As will therefore be appreciated, the present invention provides formeans for detecting the occurrence of an elementary impulsiveinterference event by identifying a sequence of perturbation to thereceived data characteristic of the effect of applying a fast voltagestep to the input of the receiver system. Particularly advantageously,means are provided for characterising the data perturbation as expectedfrom an elementary impulsive interference event by an automatic learningprocess performed, for example, when the digital receiver is firstcommissioned or each time it is switched on.

A means for reducing the effect of an elementary impulsive interferenceevent by subtracting a scaled version of the data perturbation sequencerespected from such an event is therefore achieved.

It will of course be appreciated that the present invention can readilybe incorporated into integrated circuits provided for demodulating DVB-Tsignals in order to reduce the effect of such interference.

The invention thus provides for a receiver with improved immunity toimpulsive interference events and so finds ready use particularly in DABand DVB-T systems.

The invention is described further hereinafter, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 represents a schematic block diagram of a known digital receiversystem and so as to illustrate the manner in which an impulsiveinterference event arises; and

FIG. 2 is a schematic block diagram of a receiver arrangement accordingto an embodiment of the present invention.

Turning first to FIG. 1 there is illustrated a block diagram of atypical digital receiver arrangement 10 for receiving digital wirelesssignals and which comprises an antenna 12 connected to a front-endsystem 14, which typically includes tuning and filtering devices. Thefront-end system 14 is in turn connected to analogue-to-digitalconversion means 16, which delivers an output digital signal to theremainder of the demodulation system 18.

In turn, this remainder of the demodulation system 18 provides for adigital data output 20.

The front-end system 14 serves to translate the desired range offrequencies to a required intermediate frequency range and also serve toattenuate other frequencies.

Also illustrated within FIG. 1 is an incoming impulsive interferenceevent, which could disadvantageously result in one or more bursts ofpulses at the output of the front-end system 14. It should beappreciated that each pulse within a burst is usually caused by a fastinterference voltage edge received at the antenna 12 and an example ofsuch a fast interference voltage edge 22 is also illustrated in FIG. 1.Such edges and their consequences comprise elementary impulsiveinterference events.

Although a signal edge, or a train of edges, might appear at the antenna12, each will produce a waveform 24 such as that illustrated at theoutput side of the front-end system 14 in FIG. 1. The amplitude andshape of this waveform 24 is determined by the time response of theantenna 12 and the front-end system 14. The waveform 24 can be thoughtof as comprising an impulse wavelet. Such an impulse wavelet 24 willarise in addition to the desired signal received at the antenna and atthe output of the analogue-to-digital converter 16, the voltage edge 22received at the antenna 12 has by then served to create a sequence ofdata perturbation values 26 of the wavelet 24 such as those alsoillustrated in FIG. 1.

In practice, such data perturbation values 26 will be complex in natureand so have real and imaginary parts and will also be dependent upon thesampling process. In principle however, the additional impulse waveletmaybe identifiable within the data and, once it has been assessed, itcan be subtracted from the received signals to provide for improvedaccuracy of reproduction of the received signal.

Turning now to FIG. 2, there is illustrated a schematic block diagram ofa receiver arrangement 28 according to one embodiment of the presentinvention.

The receiver arrangement 28 of FIG. 2 includes some of the typicalfeatures illustrated in FIG. 1 and so, where appropriate, commonreference numerals have been employed.

Thus, it will be appreciated that the receiver arrangement embodying thepresent invention also comprises an antenna 12, front-end system 14, ananalogue-to-digital converter 16 and a block 18 representing theremainder of the demodulation system for providing a digital data output20.

Importantly however, the embodiment of the present invention hasadditional features as discussed below.

An impulse wavelet learning system 30 is included for tapping into theoutput of the analogue-to-digital converter 16 so as to form an estimateof the shape of an impulse wavelet 24 likely to arrive at theanalogue-to-digital converter 16. The operation of such a learningsystem 30 can include the application of a known test signal via asummation device 32, which serves to receive signals from both theimpulse wavelet learning system 30 and also the antenna 12, and thendeliver an input to the front-end system 14. The known test signal canbe created at the time that the arrangement 28 is designed, orcommissioned, or indeed each time the arrangement 28 is switched on. Inthe alternative however, the wavelet shape maybe predicted from theknown characteristics of the arrangement 28, or by some otherappropriate means. However the irrespective manner in which the estimateof the shape of the impulse wavelet is obtained, once created, theestimate of the wavelet is stored within a memory unit 34.

During operation of the arrangement 28 of FIG. 2, any perturbation inthe received data due to an impulsive edge 22 received at the antenna 12is detected and assessed by an impulse wavelet detection and assessmentsystem 36 which, as with the impulse wavelet learning system 30, tapsinto the input of the analogue-to-digital converter 16. Such detectionand assessment system 36 is arranged to compare continually the receiveddata with the impulse wavelet shape stored in the memory 34. In thismanner, the detection and assessment system can employ a continuouscross-correlation, or optimal filtering, circuit. Once having detectedan impulse wavelet perturbation within the signal output from theanalogue-to-digital converter 16, the detection and assessment system 36serves to assess the amplitude and phase of the perturbation or, ifdealing with a complex value, the real and imaginary components thereofare assessed.

Such assessment of wavelet amplitude and phase are then delivered to amultiplier 38, which is also arranged to receive an output of theestimate stored in the memory 34.

The multiplier 38 is also arranged to scale the stored wavelet shape toform a more accurate estimate of the identified interference pulse whichis then subtracted from the incoming data within a summation device 40so as to produce a less-corrupted data sequence which is then deliveredto the remainder of the demodulation system 18.

Thus, as will be appreciated, the present invention provides for arelatively simple and effective means for producing a digital receiversystem in which a fast interference voltage edge received at theantenna, such as an elementary impulsive interference event and whichresults in an unwanted waveform added to the required signal, allows forprocessing at the impulse wavelet level and which impulsive wavelet isremoved from the front-end processed and digitised signal so as toeffectively remove the effect of the impulsive interference event.

The removal of the impulsive wavelet by means of an estimate of thatwavelet serves to enhance the accuracy of the noise suppression offeredby the present invention.

1. A digital receiver arrangement (28) comprising a tuner/demodulatorcircuit (14) and analogue-to-digital converting means (16), and furtherincluding means (34) for storing an impulse wavelet representation,means (36) for determining if an interference impulse (24, 26) ispresent in a received signal, and means (38, 40) for introducing thestored representation of the impulse wavelet to the detected receivedimpulse (24, 26) so as to counteract the effect thereof within thereceived signal.
 2. An arrangement as claimed in claim 1, wherein themeans (36) for determining if an impulse arises comprises comparisonmeans for comparing the stored impulse wavelet with a wavelet (24, 26)arising in the received signal.
 3. An arrangement as claimed in claim 2,wherein the comparison means (36) comprises a cross-correlator.
 4. Anarrangement as claimed in claim 2, wherein the comparison means (36)includes optimal filtering means.
 5. An arrangement as claimed in anyone of claims 1-4, wherein the means for introducing the storedrepresentation to the received signal includes subtractor means (40) forsubtracting the stored wavelet representation from the incoming impulsewavelet (24, 26).
 6. An arrangement as claimed in any one of claims 1-5,and including means (30) for determining the likely form of impulsewavelet and for introducing such likely form to the said means forstoring an impulse wavelet representation.
 7. An arrangement as claimedin claim 6, wherein the estimate of the shape of the impulse wavelet iscreated by means of a test signal.
 8. An arrangement as claimed in anyone of claims 1-5, wherein the means (34) for storing the impulsewavelet is arranged to receive a pre-programmed representation of thewavelength.
 9. An arrangement as claimed in any one of claims 1-5, andincluding prediction means for predicting the likely shape of an impulsewavelet for storage within the said means for storing.
 10. Anarrangement as claimed in any one of claims 1-9, and including means(38) for scaling the stored impulse wavelet having regard tocharacteristics of the impulse wavelet within the received signal. 11.An arrangement as claimed in claim 10, wherein the said characteristiccomprises at least one of the amplitude and phase of the impulse waveletwithin the received signal.
 12. A method of receiving a digital signalincluding the steps of demodulating the signal, and conducting ananalogue-to-digital conversion of the signal, and including the furthersteps of storing an impulse wavelet representation, determining if animpulse interference event is found within an incoming signal, andintroducing the said stored wavelet representation to the receivedimpulse interference event so as to counteract the effect thereof.
 13. Amethod as claimed in claim 12, wherein the said step of determiningincludes comparing the stored wavelet representation with a waveletwithin the received signal.
 14. A method as claimed in claim 12 or 13,and including the step of subtracting the stored wavelet representationfrom the received impulse interference event.
 15. A method as claimed inclaim 12, 13 or 14 and including the step of estimating the waveletrepresentation to be stored.
 16. A method as claimed in claim 12, 13, 15or 15 and including the step of scaling the stored waveletrepresentation responsive to characteristics of the received signal.